The present invention relates to a pneumatic brake booster for motor vehicles comprising a booster housing, which is subdivided into at least one working chamber and at least one vacuum chamber by at least one axially movable wall to which a pneumatic differential pressure can be applied, a control valve controlling the differential pressure and being arranged in a control housing in order to connect the working chamber to the vacuum chamber or to the atmosphere, the said valve being composed of two concentrically arranged sealing seats as well as an elastically deformable valve member, an operable input member, which is connected to a valve piston whose movement in an axial direction is limited by a transverse member that can be introduced in a radial direction into the control housing, an elastic reaction element, an output member applying a boosting force to a master cylinder as well as ventilating and bleeding ducts, which are provided in the control housing and allow evacuating and ventilating the working chamber.